Having trouble rendering fire+smoke with transparency
Use denoise node in compositing instead of the one you are using. It is better for viewports only.
To use denoise node,go into the compositing tab and SHIFT+A to add denoise node. Now add color,normal and albedo in their respective channel.
To get better results,enable denoising data from the layer settings.
Disable the denoiser you are using and it will give better results